Such probes are routinely used in conjunction with measuring instruments. For example, capacitive filling-level limit switches typically have a probe with at least one electrode arranged therein. The measuring instrument is arranged, for example, at the height of a predetermined filling level. An electrode serving as a measuring electrode forms a component part of a measuring capacitor, the capacitance of which depends on whether or not the probe is covered by a filling medium. The counterelectrode is formed by a separate electrode, arranged outside the probe, or by a wall of the vessel in which the probe is arranged.
Capacitive filling-level limit switches often have a second electrode, namely a shielding electrode. The shielding electrode is kept constantly at the same AC voltage potential as the measuring electrode. This prevents disturbing influences, such as contamination or built-up deposits, having an effect on the capacitance of the measuring electrode and consequently on the measurement result.
A capacitive switch of this type is described, for example, in DE-A 195 36 198.
Capacitive filling-level limit switches may additionally have a third electrode. This may likewise serve as a counterelectrode and is preferably designed as a ground electrode, in that it is connected to a ground potential or to a neutral point of a circuit of the limit switch.
DE-C 27 44 820 describes a probe having
an essentially rod-shaped probe housing, PA1 an essentially rod-shaped probe housing, PA1 an essentially rod-shaped probe housing, PA1 a clamping cone which is compressible in the radial direction and
which has an axial interior space, PA2 in which a sheet-like electrode is arranged. PA2 which has an axial interior space, PA2 in which a sheet-like electrode is arranged. PA2 which has an axial, conical interior space, PA2 is arranged in the interior space and PA2 on which there is arranged at least one sheet-like electrode,
The probe described there is produced by initially encapsulating a metal tube in a layer of plastic on the inside and outside by injection molding. A probe electrode is connected to a metal wire and pushed onto an insulator. The insulator is subsequently screwed onto a threaded portion of the metal tube and the entire arrangement is encapsulated in plastic.
The production of this probe comprises many working steps and, due among other reasons to the injection-molding operation required twice, is complex and costly.
DE-C 33 28 210 likewise describes a probe having
The probe housing described comprises a tubular main part and a cup-shaped cap. Three electrodes are arranged in the interior of the housing. A first electrode has the form of a flat pot fitted snugly in the cap and serves as a measuring electrode. A second electrode and a third electrode are respectively tubular and serve as a shielding electrode and a ground electrode. The electrodes are spaced apart from one another and electrically insulated with respect to one another in the housing by spacers. The two housing parts, the main part and the cap are securely connected to each other by ultrasonic welding.
Although the two-part housing means that it is possible to dispense with encapsulation of the electrodes, instead the housing parts must be connected to each other in a sealed manner. The connection is complex and will always represent a mechanical weakness.
In the case of the two prior-art probes described above, electrodes are stable components which at least partially lend the housing its mechanical strength.